1. Field of the Invention
The present invention generally relates to a multimedia broadcast/multicast service (MBMS). More particularly, the present invention relates to a method and apparatus for maintaining congestion and a signal transfer load of a cell at a suitable level when a session of a MBMS service is stopped in a frequency layer convergence (FLC) situation in which the MBMS service is provided in a preferred frequency layer (PL).
2. Description of the Related Art
Currently, mobile communication systems using a wideband code division multiple access (W-CDMA) scheme are actively being pursued for use in systems for multimedia broadcast/communication capable of providing not only a conventional voice service, but also a multimedia service and a packet communication service for transmitting large amounts of data. To support the multimedia broadcast/communication, a multimedia broadcast/multicast service (MBMS) service capable of being provided from one or more multimedia data sources to a plurality of user equipments (UEs) is being discussed.
The term “MBMS service” indicates a service for transmitting the same multimedia data to a plurality of receivers through a wireless network. In this case, radio transmission resources can be efficiently used because the receivers share one radio channel. This type of MBMS service supports the transmission of multimedia such as realtime video and voice, still images, text, and so on, serves as a service capable of simultaneously transmitting voice data and video data according to a type of the multimedia transmission, and requires a large amount of resources for transmission. Because a MBMS service must transmit the same data to a plurality of cells in which users are located, a Point-to-Point (PP) or Point-to-Multipoint (PM) connection is made according to the number of users located in each cell.
FIG. 1 schematically illustrates nodes joining a MBMS service in a mobile communication network. In FIG. 1, an example of a structure is illustrated in which the MBMS service is applied to the third-generation project partnership (3GPP) system corresponding to the 3G asynchronous mobile communication standard, which is incorporated by reference, based on a global system for mobile communications (GSM) and general packet radio services (GPRS).
Referring to FIG. 1, UEs 161, 162, 163, 171, and 172 represent terminal devices and subscribers capable of receiving the MBMS service. Cell-1 160 and Cell-2 170 are physical or logical service areas covered by base stations, that is, Node Bs, for transmitting MBMS data to the subscribers wirelessly. A radio network controller (RNC) 140 controls the cells 160 and 170, selectively transmits multimedia data to a specific cell, and controls a radio channel established to provide MBMS services. A connection between the RNC 140 and the UEs 161 to 172 is achieved by a radio resource control (RRC) interface.
The RNC 140 is connected to a packet switched or packet service (PS) network such as the Internet and so on by a serving GPRS support node (SGSN) 130. Communication between the RNC 140 and the PS network is achieved by packet switched (PS) signaling. Specifically, a connection between the RNC 140 and the SGSN 130 is referred to as an Iu-PS interface. The SGSN 130 controls a MBMS service for subscribers. For example, the SGSN 130 is responsible for managing data relating to service charges for each subscriber and selectively transmitting multimedia data to the specific RNC 140.
A transit network (NW) 120 provides a communication path between a broadcast/multicast-service center (BM-SC) 110 and the SGSN 130, and can be connected to an external network through a gateway GPRS support node (GGSN) (not illustrated). The BM-SC 110 serves as a MBMS data source and is responsible for scheduling the MBMS data.
On the other hand, the RNC 140 is connected to a circuit switched (CS) network by a mobile switching center (MSC) 150. The CS network is a legacy communication network for voice that is connection oriented. Communication between the RNC 140 and the MSC 150 is achieved by CS signaling. Specifically, a connection between the RNC 140 and the MSC 150 is referred to as an Iu-CS interface.
A MBMS data stream generated from the BM-SC 110 arrives at the UEs 161, 162, 163, 171, and 172 through the transit network 120, the SGSN 130, the RNC 140, and the cells 160 and 170.
Although not illustrated in FIG. 1, a plurality of SGSNs for one MBMS service and a plurality of RNCs for each SGSN can be provided. Each SGSN transmits selective data to the RNCs, and each RNC transmits selective data to the cells. For this, nodes store a list of low-level nodes to which a data stream is transferred. That is, the SGSN stores a list of RNCs and the RNC is a list of cells. Then, the nodes transmit selective MBMS data to the stored listed nodes.
FIG. 2 illustrates a procedure between a subscriber terminal and a network for a MBMS service. A BM-SC 206 communicates with a UE 202 through a RNC 204. The RNC 204 communicates with the BM-SC 206 through a SGSN (not illustrated). Here, one RNC 204 and one UE 202 are only illustrated. Of course, the same procedure can be applied to a plurality of UEs receiving the MBMS service and a plurality of RNCs for controlling the UEs.
Referring to FIG. 2, a service subscription step 210 is a process for registering the user, such as the UE 202, desiring to receive the MBMS service in the BM-SC 206 serving as a service provider. In the service subscription step 210, the service provider (BM-SC) 206 and the UE 202 exchange basic information associated with service charges or service reception.
In a service announcement step 220, UEs identify basic information for MBMS services. For example, the UEs identify MBMS service identifiers (IDs), service start times, service durations, and so on to distinguish the MBMS services capable of being provided from the BM-SC 206. Here, the MBMS service ID can be configured by a multicast address and an access point node (APN).
In the service announcement step 220, nodes located between the BM-SC 206 and the UE 202, such as the RNC 204, the SGSN, the transit network, and so on, identify the UE 202 and nodes coupled to the UE 202. For example, the SGSN identifies a list of low-level UEs desiring to receive the MBMS service and a list of low-level RNCs at which UEs are located. The SGSN refers to the lists and then transmits MBMS data only to the RNCs at which the UEs are located.
When obtaining basic information for the MBMS service, the UE 202 performs a service joining step 230 to receive MBMS service data of interest. In the service joining step 230, the UE 202 transfers, to the BM-SC 206, at least one desired MBMS service ID of MBMS service IDs obtained through the service announcement step 220.
A service notification step 240 refers to paging the UE 202 to notify the UE 202 that a session of a MBMS service joined by the UE 202 has begun started and the MBMS service will soon be started. In the service notification step 240, group paging is performed for a plurality of UEs joining the MBMS service.
A radio resource allocation step 250 refers to allocating radio resources between the UE 202 and the RNC 204 and notifying relative nodes of allocation information to provide the MBMS service. In step 250, the RNC 204 can determine the PM or PP connection on the basis of information about the number of UEs belonging to each low-level cell and a radio resource management (RRM) function.
In data transfer step 260, MBMS data is transferred to the UE 202 through the RNC 204. When information associated with the MBMS service, for example a ciphering key, needs to be changed while the MBMS data is transmitted, the RNC 204 includes new information associated with the MBMS service in MBMS control information and transfers the MBMS control information to all UEs receiving the MBMS service.
When the session of the MBMS service is stopped or terminated, the allocated radio resources are released in a radio resource release step 270 and the UE 202 is notified that the MBMS radio resources have been released. While the MBMS service is received in step 250, the UE 202 can voluntarily request reception termination of the MBMS service and can stop reception of the MBMS service.
Recent MBMS systems are supporting FLC technology for allocating a preferred frequency layer (PL) for each of the MBMS services capable of being provided and allowing UEs to receive desired MBMS services in the PL if possible. PL information indicates PLs mapped to MBMS services.
FIG. 3 illustrates an example of a cell structure in a FLC situation in a MBMS system. Under a Node B 300, three cells 301, 302, and 303 using different frequencies (Frequencies 1 through 3) spatially overlap with each other. In the cells 301 through 303, UEs 311, 312, 313 and 314 use the MBMS service in a cell on which they are camped. Frequency Layer 2 mapped to Cell-2 302 of the cells 301 to 303 is established as a PL for the MBMS service.
When a session of the MBMS service is stopped in the FLC situation as illustrated in FIG. 3, the UEs receiving the MBMS service in the PL may stay in a PL cell (in other words, Cell 2 of FIG. 3) through a cell reselection process without dispersing to other cells (in other words, non-PL (NPL) cells). In this case, because the UEs wait for a new session that may or may not start for a long time although MBMS transmission is not present on a radio link, the PL cell becomes very congested as compared with other cells and quality of service becomes degraded.
If the NPL cells have already been congested when the UEs disperse to the NPL cells due to the session stop, there is a problem in that the congestion of the NPL cells becomes worse due to the dispersion of the UEs and therefore significant intercell load may occur.